Lens

anatomy,development and physiology of eye lens

1. LENS-ANATOMY AND PHYSIOLOGY VISHNU NARAYANAN M.R

2. ANATOMY 1. Gross anatomy Biconvex, transparent, crystalline structure Ability to change shape divides eye into anterior and posterior segments 2 surfaces-anterior and posterior 8/21/2012 2

3. 8/21/2012 Lens Zonules Zonules ciliary body ciliary body 3

4. Position 8/21/2012 4

5. LOCATION OF LENS 8/21/2012 5

6. DIMENSIONS OF LENS 10 mm 6 mm 8/21/2012 6

7. Histology of lens CAPSULE ANTERIOR EPITHELIUM LENS FIBRES 8/21/2012 7

8. A)Capsule Acellular elastic structure basement membrane -type 4 collagen+sulphated GAG Variable thickness Zonules run from ciliary processes and fuse onto outer layer of capsule Main function is to mold the shape of the lens in response to tension from zonules 8/21/2012 8

9. B)Anterior epithelium Single layer of nucleated epithelial cells interconnected with gap junctions and desmosomes The only metabolically active part of lens Functionally divided into 2 zones- i] Pre-equatorial zone- columnar cells Actively dividing and differentiating into lens fibers i i]Central zone-cuboidal cells i) Transports solutes between lens and aqueous ii) Secretes capsular material Maintains the osmotic balance of lens 8/21/2012 9

10. C)Lens substance Composed of lens fibres Consist of primary and secondary fibers Produced by mitosis of epithelial cells in the pre-equatorial zone They elongate and undergo differentiation with pyknocytosis and eventual loss of cell oraganelles and nucleus This is an important factor in the transparency of the lens 8/21/2012 10

11. As lens fibres elongate and new ones form, the older ones are pushed towards the depth of the lens Lens bow-shifting of lens nuclei forms a visible line in the equator Fibres are arranged in zones in which fibres growing from opposite directions meet in sutures Consist of nucleus and cortex 8/21/2012 11

12. LENS FIBRES-PHOTOMICROGRAPH 8/21/2012 12

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15. Cortex Youngest lens fibres It is located peripherally, and is composed of secondary fibres formed continuously after puberty 8/21/2012 15

16. Nucleus Central part with oldest fibres Dense and compactly arranged lens fibres Higher refractive index than capsule Different zones depending on period of development 8/21/2012 16

17. epinucleus 8/21/2012 17

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19. 8/21/2012 Anterior Suture ( erect Y ) posterior suture inverted Y 19

20. Sutures 8/21/2012 20

21. Zonules of zinn Suspensory ligaments/ ciliary zonules Series of fibres from ciliary process Holds the lens in position Assist action of ciliary muscle Attached to lens capsule at zonular lamella 8/21/2012 21

22. Parts of ciliary zonules 8/21/2012 22

23. Types of zonules 8/21/2012 23

24. Zonular spaces 8/21/2012 24

25. EMBRYOLOGY 1)Formation of lens vesicle 4 weeks Optic vesicle induces lens placode from ectoderm Lens placode invaginates and becomes lens pit Optic vesicle also invaginates and becomes optic cup Lens pit separates from ectoderm to become the lens vesicle 8/21/2012 25

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27. 2)Formation of lens fibers and zonules Primary lens fibres—fibres formed upto 3 rd month Cells in posterior portion of lens vesicle elongate to fill vesicle forms emryonic nucleus Secondary lens fibres—3 rd month to entire life Cells in anterior portion of vesicle divide actively and elongate includes all other nucleus Lens capsule-produced by anterior epithelial cells Lens zonules —from neuroectoderm in ciliary area(3 rd – 5 th month) Tunica vasculosa lentis --nourishment to embyonic lens branch of hyaloid artery disappears before birth 8/21/2012 27

28. PHYSIOLOGY OF LENS COMPOSITION 8/21/2012 28

29. LENS PROTEINS Water soluble lens crystallins 90% of total lens protein 1) Alpha crystallin Largest crystallin Accounts for 31% total lens protein 2) Beta crystallin Most abundant crystallin , accounts for 55% total lens protein Most heterogenous group, 6 distinct subgroups 3) Gamma crystallin Smallest crystallin Least abundant-2% Water insoluble proteins Insoluble albuminoids-12% 8/21/2012 29

30. WATER Lens –dehydrated state Unique arrangement of proteins within lens fibres Low protein osmotic activity within lens Tightly packed fibres with minimum extracellular spaces Lens epithelium transports water into the fibre mass Half of the water -protein hydration Water excreted via aquaporin in the equator into aqueous Important factor maintaining lens transparency 8/21/2012 30

31. Electrolytes Active transport of ions and low molecular weight metabolite takes place between lens and aqueous humour Epithelium contains Na-K-ATPase and a calmodulin - dependent Ca -activated ATPase for the active transport of electrolytes Fibre cells contain large concentrations of negatively charged crystallines Positively charged cations enter the lens cell to maintain electrical neutrality pH- 6.9-7.2 Aminoacids transported in lens via energy dependent carrier mechanisms 8/21/2012 31

32. Lipids-high concentration of cholesterol and sphingomyelin membrane rigidity Glutathione –major antioxidant in lens synthesized by GSH in lens present in reduced state Ascorbic acid-synthesized by cililary body into aqueous antioxidant property Inositol- osmolyte membrane rigidity Taurine - osmolyte antioxidant 8/21/2012 32

33. Pump-leak mechanism 8/21/2012 33

34. Functions of lens Refraction Accounts for 35% of total refractive power of eye (15D out of total of 58D) Light transmission Focusing of visible light rays on the fovea Preventing , damaging- ultra-violet radiation, from reaching the retina Accomodation Organizer of anterior segment 8/21/2012 34

35. REFRACTIVE PROPERTIES 8/21/2012 35

36. ACCOMODATION Lens has the capacity to change the focussing power of the eye for distant and near vision Accomodation can be divided into physical process- measure of change in shape of lens physiological process- measure of ciliary muscle contraction Near reflex- Contraction of ciliary muscles Contraction of pupils Convergence of visual axis 8/21/2012 36

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38. Theories of accomadation 1)Helmholtz theory - ciliary muscle contraction relaxation of zonules lens-spherical aging—lens rigid—difficulty to change shape Classical theory 8/21/2012 38

39. 2) S chachar’s theory– ciliary muscle contraction equatorial zonules tensed shape changes in lens Aging—diameter of lens grow—less space for proper functioning of ciliary muscles 3) Coleman’s theory— Lens and zonules —diaphragm Shape altered by pressure difference b/w aqueous and vitreous Ciliary muscle-alters pressure gradient 8/21/2012 39

40. Maintenance of transparency Thin epithelium Regular arrangement of lens fibers Little cellular organelles Little extracellular space Orderly arrangement of lens proteins Relative dehydration Semipermeable character of lens capsule Avascularity Antioxidants 8/21/2012 40

41. LENTICULAR METABOLISM Continous supply of ATP required for- Transport of ions and aminoacids Maintanence of lens dehydration Continous protein synthesis GSH synthesis Major site – epithelium Source of nutrient supply-aqueous humour 8/21/2012 41

42. Glucose metabolism Energy production entirely dependent on glucose metabolism Glucose enters lens by simple diffusion and facilitated diffusion Epithelial cells- GLUT-1 Lens fibre cells-GLUT-3 Glucose is rapidly metabolized via glycolysis so that level of free glucose in lens < 1/10 level in aqueous 8/21/2012 42

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44. 1)Anaerobic metabolism Accounts for 85% of glucose metabolism by lens Provides > 70% of energy for lens 1 mole of glucose gives 2 moles of ATP Lactate generated undergoes 2 pathways of metabolism Further metabolism via Kreb’s cycle Diffusion from lens into aqueous 2)Aerobic metabolism (Krebs cycle) Limited to epithelium 1 mole of glucose gives 38 moles of ATP Only 3% of lens glucose metabolized by this pathway But generates up to 20% of total ATP needs of lens 8/21/2012 44

45. 3)Hexose monophosphate shunt Accounts for 5% of glucose metabolism by lens Important source of NADPH required for other metabolic pathways e.g. sorbitol pathway and glutathione reductase 4)Sorbitol pathway Accounts for 5% of glucose metabolism by lens When sorbitol accumulates within cells of lens, it sets up an osmotic gradient that induces influx of water and lens swelling, and ultimate loss of lens transparency 8/21/2012 45

46. Changes during aging 1)Changes in structure Crystallins — proteolysis,decresed solubility,aggregation racemization and deamidation of aminoacids Cytoskeleton—proteolysis and insolubilization —disassembly of fibres Leads to opacities –nuclear sclerosis—senile cataract 2)Less elasticity of lens — loss of power of accomodation —presbyopia 3)Overall reduction in light transmission 8/21/2012 46

47. DISEASES OF LENS 1)CONGENITAL DISORDERS Coloboma Lenticonus Lentiglobus Microspherophalus Mittendorf dot 2)CATARACTS Congenital Acquired 8/21/2012 47

48. 3) Miscellaneous disorders Posterior capsular opacification Aphakia Pseudophakia Exfoliation of capsule Psuedoexfoliation and psuedoexfoliation glaucoma Lens induced glaucoma Ectopia lentis Lens induced uveatits Loss of accomodation-aging,cataract,surgery 4)Several systemic diseases are also associated with disorders of the lens 8/21/2012 48

49. THANK YOU!!!!